Accelerator Facility Accident Report
31 May 2013

Incorporated Administrative Agency - Japan Atomic Energy Agency
Inter-University Research Institute - High Energy Accelerator Research Organization

Subject: Radioactive Material Leak at the Hadron Experimental Facility of Japan Proton
Accelerator Research Complex (J-PARC)

Time and Date of Accident: Approximately 11:55 am on 23 May 2013

Accident Location: Hadron Experimental Facility, J-PARC

Description of Accident:
It is presumed that due to a malfunction of the beam extraction system, the gold target of theHadron Experimental Facility (hereafter HD Facility) was damaged and radioactive material wasdischarged into the room that contained the target and the primary beamline. The radioactive
material then leaked into the experimental hall where workers were exposed to radiation.

Area monitors around the boundary of the controlled area registered an increase in ambient
radiation. Higher than normal levels were also observed for a short period at the three monitoring
posts and stations near to J-PARC of the neighboring Nuclear Fuel Cycle Engineering
Laboratories. Accordingly, it is reasoned that there was a radiation leak from within thecontrolled area of the HD Facility.

Radiation Leakage and Exposure within the Controlled Area:
It is presumed that the gold target of the HD Facility was damaged and radioactive material
was discharged into the room that contained the target and the primary beamline. The
radioactive material then leaked into the experimental hall.

Thirty four (34) registered radiation workers in the controlled area of the HD Facility received
total (internal plus external) radiation doses between 0.1 and 1.7 mSv.

Radiation Leakage into the Environment and It’s Effects:
Simulations indicate that the leaked material would have diluted and decayed as it dispersedin a narrow range towards the west of the HD Facility. It has been estimated that the maximum1integrated radiation dose at the site boundary closest to the HD Facility was 0.29 µSv(preliminary).

Cause of the Accident:
A detailed investigation will be carried out.

Immediate Response and Future Measures:
All J-PARC accelerators have been shut down and access to the hadron experimental hall
has been restricted. Taking into account the findings of the investigation, all necessary
measures to prevent a reoccurrence of the accident will be examined.

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Supporting document1. Subject
Radioactive Material Leak at the Hadron Experimental Facility of Japan Proton Accelerator
Research Complex (J-PARC)

2. Time and date of accident
Approximately 11:55 am on 23 May 2013

3. Accident location
Hadron Experimental Facility, J-PARC

4. Circumstances of the accident
4.1 Operation status at the accident
The Hadron Experimental Facility (hereafter HD Facility) (Fig. 1) of J-PARC, completed in
January 2009, is located at the south end of the site of the Nuclear Science Research Instituteof the Japan Atomic Energy Agency (JAEA).

In the Hadron experimental hall, the proton beam extracted from the 50 GeV Synchrotron(hereafter MR) is brought onto a gold target to produce secondary particles, such as K-mesons,π-mesons and others. These secondary particle beams are transported to several
experimental areas in the hall for carrying out various experiments. The entire HD Facility
consists of the experimental hall, the power supply building and the mechanical support
building (Fig. 2). The plan view of the experimental hall is shown in Figure 3.

At around 11:55 of 23 May, due to a malfunction of the beam extraction system of the MR, abeam consisting of 2 x 10 13 protons was delivered to the gold target (Fig. 4) within a very shorttime (5/1000 of a second), whereas normally a total of 3 x 10 13 protons were slowly and evenly
extracted and delivered to the target over a period of 2 seconds. As a result, the gold target isconsidered to have momentarily reached an extremely high temperature and possibly to havebeen partially damaged.

Fig. 4 shows the design of the gold target. It is a square rod of gold (W6 mm x H6 mm x L66mm) which is mounted directly on top of a copper block which is water-cooled for removal ofheat. Thermocouples are used to monitor the target temperature. This target assembly ishoused in a metal container as shown in Fig.5.

Major events from 23 to 25 May are chronologically listed in the appendix.

3(1) Prior to the accident
The proton beams had been delivered from MR to the Hadron experimental hall since 13May. The operation mode on 23 May involved the delivery of 3 x 10 13 particles per 2 seconds.

(2) At the accident
At around 11:55, the Machine Protection System (MPS) that controls beam delivery to theexperimental hall detected an anomalous condition and the beam delivery was halted. The shift
leader of MR, the person in charge of the power supply, and the Hadron beam operator on shiftinspected the situation and reset the MPS. One test shot of the beam was delivered to thetarget to confirm the normalcy of the beam trajectory. The delivery of proton beam wasresumed at around 12:08.

At around 13:30, it was acknowledged that the readings of gamma-ray area monitors in theexperimental hall were increased to 4µSv/hr, 10 times higher than the normal. The person in
charge of the beam production facility and the operator on shift discussed, over the phone, on
the situation and ways to identify the cause. They noted that the health of the monitors could beconfirmed by turning on ventilation fans mounted on building walls. If the radiation rate isreduced with the use of ventilation fans, the cause could be radioactive air. Otherwise, it could
be due to problems in the beam trajectory or a malfunction of the area monitor. They turned on
the ventilation fans between ~15:15 and 15:32.

At around 15:32, they recognized reduction in the radiation dose rate and then resumed thebeam delivery for retuning the beam. However, as the dose rate increased again, they stopped
the beam at around 16:15.

The person in charge of the radiation controlled area of the HD Facility discussed the issue
with the radiation protection supervisor on the phone. Since the radiation dose rate in the hall
was sufficiently lower than the does limit value of 25µSv/hr as stipulated by regulations(equivalent to the dose limit of 1mSv/week), they assumed no environmental effects to theareas outside the radiation controlled area by turning the fans on. Hence, they turned theventilation fans on again to further reduce the radiation dose rate in the hall at around 17:30.

(3) After the accident
At around 17:30 the next day, 24 May, J-PARC received an inquiry from the Nuclear Fuel
Cycle Engineering Laboratories that momentary rises in the radiation dose rate were recordedby their three monitoring posts and stations which are located near the HD Facility. The datalogs of gamma-ray monitoring posts on the external boundary of the radiation controlled area4

of the HD Facility were thoroughly reviewed. J-PARC found that radiation dose rate increased
at around 15:00 and 17:30 on 23 May and that the increases coincided with the operation ofventilation fans in the experimental hall. They recognized for the first time a high possibility ofleakage of radioactive materials to the areas outside of the radiation controlled area of the HD
Facility.

At 21:10, J-PARC reported on this incident to the emergency number of the Nuclear
Science Research Institute. The local emergency response headquarters was immediatelyestablished. At around 22:15, the headquarters confirmed that a statutory report was due.
JAEA reported to the Nuclear Regulation Authority at 22:40 in accordance with laws andregulations, and reported to Ibaraki Prefecture, Tokai Village, and neighboring local
governments using fax in accordance with agreements with local governments.

4.2

Reporting

As of 23 May, the radiation protection supervisors had recognized that: part of the gold target
was damaged, radioactive material leaked into the hadron experimental hall, the floor and otherareas were contaminated, and workers might have suffered internal radioactive exposure.
However, they considered that the contamination was localized in the radiation controlled area,
and the exposure was within the expected scope, and thus they evaluated that statutory
reporting was not due.

At around 17:30 on 24 May, a report was received on momentary increases in the radiationdose rate at the three monitoring posts and stations near J-PARC of the Nuclear Fuel Cycle
Engineering Laboratories. This was when a high probability of radioactive material leakage
outside of the controlled area was noted for the first time, resulting in a report to the emergency
number of the Nuclear Science Research Institute at 21:10 on 24 May.

Reporting was delayed due to these circumstances.

5. Radiation leakage and exposure within the radiation controlled area
Due to a malfunction of the beam extraction system of the MR, at around 11:55 the proton
beam was brought onto the target in a time period (5/1000 of a second), much shorter than thenormal two seconds. As a result, the gold target is considered to have momentarily reached an
extremely high temperature and possibly to have been partially damaged, and discharge ofradioactive material resulted in the room that contained the target assembly. Radioactive
material then leaked into the hadron experimental hall. Workers were exposed to radiation in thehadron experimental hall due to this leakage.

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A total 102 individuals entered the radiation controlled area of the HD Facility after theoccurrence of this accident. Except for two users from overseas who have returned to their homecountries, radiation dose measurements have been conducted on all of the remaining 100individuals. Exposure has been confirmed for 34 registered radiation workers. Their total(internal plus external) radiation doses were between 0.1 and 1.7 mSv. This does not exceed thedose limit for registered radiation workers stipulated by law. The two users who returned to theirhome countries are scheduled to have measurements at medical institutions in their homecountries.

6. Radiation leakage into the environment and its effects
It was confirmed that readings of the area monitors, which were installed at the externalboundary of the controlled area near the hadron experimental hall, showed increases whenventilation fans were turned on (Figure 6). However, the data logs of radiation dose rates atmonitoring posts in other places in the Nuclear Science Research Institute showed no signatures.
On the other hand, at the three monitoring posts and stations of the neighboring Nuclear Fuel
Cycle Engineering Laboratories, near J-PARC, momentary rises of the radiation dose rate,
exceeding the normal range of fluctuation, were observed twice, and they were in coincidencewith the release associated with operation of ventilation fans. They indicated that it is likely thatradioactive material was allowed to leak from inside the hadron experimental hall to thesurrounding environment (outside the radiation controlled area).

The radiation dose in the surrounding environment was evaluated by using two methods: ananalytic method based on the diffusion formula for radioactive material, and the WSPEEDI-II
numerical simulation software. Inputs to these evaluations include the results of nuclide
composition analysis of air samples collected at the HD Facility, data logs of the gamma-ray
area monitors installed in the facility, meteorological data for the time period in question, andother relevant information.

It is assessed that the released radioactive material was diluted and attenuated as itdispersed in a narrow strip towards the west. It has been estimated that the maximum integrated
radiation dose due to the release of radioactive material would be 0.29 µSv at the site boundary
closest to the HD Facility. The amount of released radioactive material is under continued
evaluation.

7. Causes
The gold target was possibly partially damaged due to a malfunction of the beam extraction
system of the MR, and radioactive material leaked into the hadron experimental hall. Operationof ventilation fans caused the leak of radioactive material out of the radiation controlled area.

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A detailed investigation will ensue.

8. Response and measures
Operation of all J-PARC accelerators has been halted and access to the hadronexperimental hall has been restricted. Investigations will be conducted regarding the cause of
the malfunction of the beam extraction system, discharge of radioactive material from the target,and issues in the safety management.

On the basis of results from the investigations above, measures against reoccurrence ofsimilar accidents will be examined.

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Hadron Experimental
Facility
Pacific Ocean
The Japan Atomic
Power Company
Tokai Power Station •Tokai No. 2 PowerStation8The Japan Atomic
Energy Agency
Nuclear FuelEngineeringLaboratories------ site boundary

Figure 1: Location of Hadron Experimental Facility

boundary lines of radiation
controlled area of the Hadron
Experimental Facility

direction of the view

Figure 2: Hadron Experimental Facility

Gold target

Figure 3: Plan View of Hadron Experimental Facility (as of 2013)

9Copper block

Gold (□6mm x L66 mm)

Proton beam

Thermocouples

Cooling watertubingFigure 4: Gold Target before Installation

Beam profile
monitorContainerforthe goldtarget

Proton beam

Figure 5: Container for the gold target

10Monitor 201

8 ventilation fans

Monitor 202
11experimental hall

Spikes in red circles

8 ventilation fanscorrespond to operationof ventilation fans

Monitor 203

Figure 6: Trends of the gamma-ray rate, measured with area
monitors at boundary of the radiation controlled area of theHadron Experimental Facility

Appendix

Chronological Sequence of Events

23 May 2013
11:55

Delivery of proton beam is halted by Machine Protection System (MPS).

12:08

MPS was reset and delivery of proton beam was resumed.

12:30

An increase in the background level was noted on an instrument used for experiments(neutron detector).

13:30

An elevated radiation dose rate in the experimental hall was acknowledged. Ten-foldincrease (to 4 µSv/h) was measured with a gamma-ray monitor.

14:26

Beam was stopped. Reduction in gamma-ray dose rate was observed.

15:15

Further reduction in ambient dose rate was found when ventilation fans were turned on.

15:32

Ventilation fans were turned off and beam extraction was resumed

16:00

Higher-than-normal radiation rate in the experimental hall (4-6 µSv/h) was measured witha survey meter. An increased gamma-ray rate was also noted.

16:15

Beam operation was stopped.

17:00

Radiation survey of experimental hall found localized areas of high dose rate.

17:20

An air sample was collected in the experimental hall for measurements.

17:30

Ventilation fans were turned on to reduce airborne radiation dose rate in experimentalhall.
Evacuation of workers from the experimental hall began.

19:00~20:00 Systematic measurements of radiation dose rate and surface contamination carriedout in experimental hall.
23:30 Completed evacuation and full-body radiation surveys of all workers in radiation controlledarea. Access to area restricted.
24 May 2013
17:30

J-PARC received inquiry from Nuclear Fuel Engineering Laboratories concerning
increased radiation levels recorded by their monitoring posts at around 15:00 and 17:30
the previous day (23 May).

18:00

Data logs of gamma-ray monitoring posts on the boundaries of the controlled area of theHadron Experimental Facility were examined. Found increased radiation levels at around
15:00 and 17:30 on 23 May. Found that the increased dose rates coincide with operationof ventilation fans in experimental hall.

21:10

Report to an emergency post of the Nuclear Science Research Institute21:19

Nuclear Regulation Authority was contacted by telephone

21:40

Ibaraki Prefecture was contacted by telephone.

21:43

Tokai Village was contacted by telephone.

22:40

As required by law, the first report to the Atomic Energy Regulatory Committee wastransmitted by facsimile transmission.

22:40

As required under the terms of relevant Agreements, the first report was faxed to Ibaraki
Prefecture, Tokai Village and other authorities.

25 May 2013
01:00

Found out that the maximum radiation dose received by workers was 1.7 mSv(Provisional)